When a string contains zero-width characters, `LineBreakMeasurer` calculates line breaks incorrectly.
The root cause appears to be that `LineBreakMeasurer` eventually calls into `StandardGlyphVector.getGlyphInfo()`, which derives the glyph advances from the glyph IDs. However, HarfBuzz's default treatment of zero-width characters is to provide the glyph ID of the space character (`U+0020`) combined with an artificial zero advance (not the font's space glyph advance). Unaware of HarfBuzz's sleight of hand, `StandardGlyphVector.getGlyphInfo()` retrieves the actual advances of the space glyph (since that was the glyph ID returned) and provides these back up the call chain to `LineBreakMeasurer` et al. I think the correct fix is to use `hb_buffer_set_invisible_glyph` to register `0xFFFF` as the invisible glyph ID with HarfBuzz (matching `CharToGlyphMapper.INVISIBLE_GLYPH_ID`). I haven't seen any unwanted side effects, but there is a risk, since this is changing the global HarfBuzz configuration. For more information on HarfBuzz's behavior in this area, see: https://harfbuzz.github.io/setting-buffer-properties.html ------------- Commit messages: - Fix LineBreakMeasurer when using zero-width chars Changes: https://git.openjdk.org/jdk/pull/23603/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23603&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8270265 Stats: 14 lines in 5 files changed: 13 ins; 0 del; 1 mod Patch: https://git.openjdk.org/jdk/pull/23603.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/23603/head:pull/23603 PR: https://git.openjdk.org/jdk/pull/23603
